Heavy oils are generally referred to those oils with high viscosity or API gravity less than about 23. Crude oils and crude oil residuum derived from atmospheric or vacuum distillation of crude oil are examples of heavy oils. The origin of high viscosity in these heavy oils has been attributed to high asphaltene content and the asphaltene aggregation phenomenon. Viscosity reduction of heavy oils is important in production, transportation and refining operations of heavy oils. Fouling of metal surfaces by asphaltene containing oils is also a problem in heavy oil refining and transportation. The fouling tendency of an oil is generally evaluated by measuring the toluene equivalence (TE) number for the oil. A need exists to find effective methods to reduce viscosity and fouling tendency of oils containing asphaltenes. The instant invention addresses these needs.